This invention relates to an improved impact attenuating device adapted to be mounted on a vehicle, of the type comprising an energy absorbing element and a mounting arrangement coupled to the energy absorbing element and configured to mount the energy element on the vehicle.
Krage et al. U.S. Pat. No. 4,711,481 discloses a vehicle impact attenuating device of the general type described above. The disclosed device (which is often referred to in the industry as a truck mounted attenuator or TMA) is typically installed at the rear of a highway maintenance vehicle which in use is often parked alongside a roadway. A TMA extends rearwardly from the vehicle and is designed to collapse in an impact, thereby protecting the occupants of an impacting vehicle from injury.
The TMA of the Krage patent includes a generally rectangular box made up of sheet aluminum. Aluminum sheet diaphragms extend in parallel vertical planes across the interior of the box, and cell assemblies are interposed between adjacent diaphragms. Each cell assembly is formed of two L-shaped sheet metal components which are riveted together to form a rectangular column extending between the adjacent diaphragms. The cell assemblies are deformed in an impact, and the configuration of the cell assemblies and in particular the diagonal bracing between adjacent columns increases the energy absorbing capacity of the columns.
The Krage TMA has been found effective in use and has been commercially successful. However, the structure of the Krage TMA is essentially a single integrated module. A collision which damages any part of this module will often require that the entire module be replaced. Additionally, a vehicle having a bumper unusually close to the ground may have a tendency to dive under the end of the TMA in an impact. In the event this should happen, the stopping efficiency of the TMA is reduced.